Co‐Invasion by Distantly Related Invasive Plants Drives Invasional Meltdown via Synergistic Increases in Putative Soil Fungal Pathogens

Biological invasion is a major component of global change, and co-invasion of multiple invasive species is becoming increasingly common under accelerating globalization, climate warming, and land-use change. Such co-invasions can generate non-additive impacts on ecosystems, either exacerbating or mitigating invasion outcomes, yet their ecological consequences and underlying mechanisms remain poorly understood. We conducted a two-phase greenhouse experiment using Solidago canadensis as a focal invader and 16 co-occurring invaders to condition soils in monocultures and mixtures, and then tested effects of these soils on a native plant community. We found that non-additive effects of co-invasions on native community biomass were generally negative, with phylogenetically distant co-invaders exerting stronger negative effects. These patterns were largely driven by synergistic increases in the richness of putative soil fungal pathogens induced by distantly related co-invaders. A complementary field survey confirmed these patterns in natural communities, showing that greater phylogenetic distance among co-invaders was associated with higher richness of putative soil fungal pathogens and stronger reductions in native plant abundance. Our study provides the first empirical evidence that evolutionary relatedness among co-invaders predicts the direction and magnitude of their combined impacts via soil microbial pathways. These findings highlight the importance of incorporating evolutionary and belowground mechanisms into invasion theory, and underscore the urgent need to recognize co-invasion as a key process for predicting and managing invasion impacts under global change.